Electronic display devices are becoming an increasingly indispensable tool in modern society for the delivery of visual information. These devices find widespread utility in television sets, computer terminals, and in a host of related applications. No other type of technology offers comparable speed, versatility and potential for interactivity. Current electronic display technologies include, for example, cathode ray tubes (CRT's), plasma displays, light emitting diodes (LED's), thin film electroluminescent displays, and the like.
The most widely-used non-emissive technology for display devices makes use of the electro-optic properties of a class of organic molecules known as liquid crystals in fabricating liquid crystal displays (LCD's). LCD's operate fairly reliably, but are limited by relatively low contrast, low speed, and fade-out when viewed from oblique angles, as well as the requirement for high power backlighting. Active matrix displays, in a partial solution to these shortcomings, employ an array of transistors, each capable of activating a single liquid crystal pixel, thus improving contrast.
There is no doubt that flat panel display technology is of significant scientific and commercial interest. Consequently, it is the subject of extensive ongoing research. See Depp, S. W. and Howard, W. E., "Flat Panel Displays," Scientific American, March 1993, pps. 90-97. According to Depp and Howard, by 1995, flat panel displays alone were expected to generate a market of between $4 and $5 billion. Key to the success of any potential display technology in this market is the ability to both provide a high resolution, full-color display at good light level and, at the same time, to be competitively priced.
Organic thin film materials represent a technical development that has demonstrated considerable progress in the fabrication of red, green and blue light emitting devices. These organic light emitting devices have been shown to have sufficient brightness, range of color and operating lifetimes for use as a practical alternative technology to LCD-based full color flat-panel displays (S. R. Forrest, et al., Laser Focus World, February 1995). Furthermore, since many of the organic thin films used in such devices are transparent in the visible spectral region, they potentially allow for the realization of a completely new type of display pixel in which the red (R), green (G), and blue (B) emission layers are placed in a vertically stacked geometry to provide a simple fabrication process, minimum R-G-B pixel size, and maximum fill factor.
Disclosed in U.S. Pat. No. 5,294,869 to C. W. Tang and J. E. Littman is a concept for using separate, side-by-side red, green, and blue OLED's to make a full color display. However, it is believed by the inventors of the instant disclosure that such concepts have never been successfully realized in a practical device.
Such schemes suffer from a complex layer structure, and lack of known methods for damage-free, post-deposition patterning of organic layers at the resolution required for color displays. Others have alternatively suggested using an array of white OLED's (J. Kido, et al., Science 267, 1332 (1995)) backed by side-by-side R, G and B color filters deposited and patterned prior to OLED growth. However, such a design sacrifices at least 66% of the light from each white OLED, with the remainder being absorbed in the filter, also generating heat. Such a design suffers, therefore, from low efficiency and conditions of accelerated degradation. Alternative schemes based on microcavity filtering of a broad-spectrum OLED (A. Dodabalapur, et al., Appl. Phys. Lett. 64, 2486 (1994)) suffer from complex and expensive substrate patterning requirements and extremely limiting directionality of the resulting color pixels.
An example of a multicolor electroluminescent image display device employing organic compounds for light emitting pixels is disclosed in Tang et al., U.S. Pat. No. 5,294,870. This patent discloses a plurality of light emitting pixels which contain an organic medium for emitting blue light in subpixel regions. Fluorescent media are laterally spaced from the blue-emitting subpixel region. The fluorescent media absorb light emitted by the organic medium and, in turn, emit red and green light in different subpixel regions. The use of materials doped with fluorescent dyes to emit green or red on absorption of blue light from the blue subpixel region is less efficient than direct formation via green or red LED's. The reason is that the efficiency will be the product of (quantum efficiency for EL) and (quantum efficiency for fluorescence) and (1-transmittance). Thus, a drawback of this display and all displays of this type is that different laterally spaced subpixel regions are required for each color emitted.
Color-tunable OLED's potentially allow for full-color operation without the complex structures common to other types of devices. Published examples of tunable OLED's utilize a blend of either two polymers (M. Granstrom and O. Inganas, Appl. Phys. Lett. 68, 147 (1996)) or a polymer doped with semiconductor nanocrystallites (B. O. Dabbousi, et al., Appl. Phys. Lett. 66, 1316 (1995); V. L. Colvin, et al., Nature 370, 354 (1994)). Each component of the blend emits radiation having a different spectral energy distribution. The color is tuned by varying the applied voltage. A higher voltage results in more emission from the higher bandgap polymer, which emits radiation toward the blue region of the spectrum, while also resulting in higher overall brightness due to increased current injection into the device. Although tuning from orange to white has been demonstrated, incomplete quenching of the low-energy spectral emission appears to prohibit tuning completely into the blue. In addition, emission intensity can only be controlled by using pulsed current and reduced duty cycles. In a color display, therefore, prohibitively high drive voltages and very low duty cycles may be necessary for blue pixels. This necessitates a complex driver circuit, renders passive matrix operation extremely difficult, if not impossible, and is likely to accelerate degradation of the display.
A transparent organic light emitting device (TOLED) which represents a first step toward realizing high resolution, independently addressable stacked R-G-B pixels has been reported recently in the published international patent application No. WO 92/19792, which corresponds to U.S. Pat. No. 5,707,745. This TOLED had greater than 71% transparency when turned off, and emitted light from both top and bottom device surfaces with high efficiency (approaching 1% quantum efficiency) when the device was turned on. The TOLED used transparent indium tin oxide (ITO) as the hole-injecting electrode layer, and a Mg:Ag-ITO layer for electron injection. A device was disclosed in which the Mg:Ag-ITO electrode was used as a hole-injecting contact for a second, different color-emitting OLED stacked on top of the TOLED. Each device in the stack was independently addressable and emitted its own characteristic color through the transparent organic layers, the transparent contacts and the glass substrate allowing the entire device area to emit any combination of color that could be produced by varying the relative output of the two color-emitting layers.
Thus, for the specific device disclosed in WO 92/19792, which included a red-emitting layer and a blue-emitting layer, the color output produced by the pixel could be varied in color from deep red through blue.
It is herein believed that WO 92/19792 provided the first demonstration of an integrated OLED where both intensity and color could be independently varied by using external current sources. As such, WO 92/19792 represents the first proof-of-principle for achieving integrated, full color pixels which provide the highest possible image resolution (due to the compact pixel size) and low cost fabrication (due to the elimination of the need for side-by-side growth of the different color-producing pixels).
Presently, a frequently used high-efficiency organic emissive structure is one referred to as the double heterostructure LED which is known to those of skill in the appropriate art. This structure is very similar to conventional, inorganic LED's using materials as GaAs or InP. In this type of device, a support layer of glass is coated by a thin layer of indium/tin oxide (ITO) to form the substrate for the structure. Next, a thin (100-500 .ANG.) organic, predominantly hole-transporting, layer (HTL) is deposited on the ITO layer. Deposited on the surface of the HTL layer is a thin (typically, 50-100 .ANG.) emissive layer (EL). If these layers are too thin, there may be breaks in the continuity of the film; as the thickness of the film increases, the internal resistance increases, requiring higher power consumption for operation. The range of 100-1000 .ANG. represents the best typical compromise between these extremes. The emissive layer (EL) provides the recombination site for electrons, injected from a 100-500 .ANG. thick electron transporting layer (ETL) that is deposited upon the EL, and holes from the HTL layer. The ETL material is characterized by considerably higher mobility for electrons than for charge deficient centers (holes). Examples of prior art ETL, EL and HTL materials are disclosed in U.S. Pat. No. 5,294,870 entitled "Organic Electroluminescent MultiColor Image Display Device," issued on Mar. 15, 1994 to Tang et al., the disclosure of which is hereby incorporated by reference.
Often, the EL layer is doped with a highly fluorescent dye to tune the frequency of the light emitted (color), and increase the electroluminescence efficiency of the LED. The double heterostructure device described above is completed by depositing metal contacts onto the ITO Layer, and a top electrode onto the electron transporting layer. The contacts are typically fabricated from indium or Ti:Pt:Au. The electrode is often a dual-layer structure consisting of an alloy such as Mg:Ag directly contacting the organic ETL layer, and a thick, opaque second layer of a high work function metal such as gold (Au) or silver (Ag) on the Mg:Ag or the transparent Mg:Ag/ITO electrode. When proper bias voltage is applied between the top electrode and the metal contacts, light emission occurs through the glass substrate for devices with an opaque top electrode, and through both surfaces for transparent OLED's. An LED device of this type typically has luminescent external quantum efficiencies of from 0.05 percent to 4 percent depending on the color of emission and the device structure.
Another known organic emissive structure is referred to as a single heterostructure. The difference in this structure relative to that of the double heterostructure is that the electroluminescent layer also serves as an ETL layer, eliminating the need for the ETL layer. However, this type of device, for efficient operation, must incorporate an EL layer having good electron transport capability, otherwise a separate ETL layer must be included, rendering the structure effectively the same as a double heterostructure.
Presently, the highest efficiencies have been observed in green LED's. Furthermore, drive voltages of 3 to 10 volts have been achieved. These early and very promising demonstrations have used amorphous or highly polycrystalline organic layers. These structures undoubtedly limit the charge carrier mobility across the film which, in turn, limits current and increases drive voltage. Migration and growth of crystallites arising from the polycrystalline state is a noted failure mode of such devices. Electrode contact degradation is also a common mechanism of failure.
A known alternative device structure for an LED is referred to as a single layer (or polymer) LED. This type of device includes a glass support layer coated by a thin ITO layer, forming the base substrate. A thin organic layer of spin-coated polymer, for example, is then formed over the ITO layer, and provides all of the functions of the HTL, ETL, and EL layers of the previously described devices. A metal electrode layer is then formed over the organic polymer layer. The metal is typically Mg, Ca, or other conventionally used metals.
Devices whose structure is based upon the use of layers of organic optoelectronic materials generally rely on a common mechanism leading to optical emission. Typically, this mechanism is based upon the radiative recombination of a trapped charge. Specifically, devices constructed along the lines discussed above comprise at least two extremely thin organic layers separating the anode and cathode of the device. The material of one of these layers is specifically chosen based on the material's ability to transport holes (the HTL layer); the other, according to its ability to transport electrons (the ETL or EL layer). This last layer typically comprises the electroluminescent layer. With such a construction, the device can be viewed as a diode with a forward bias when the potential applied to the anode is higher than the potential applied to the cathode. Under these bias conditions, the anode injects holes (positive charge carriers) into the electroluminescent layer, while the cathode injects electrons into the EL layer. The portion of the luminescent medium adjacent to the anode thus forms a hole injecting and transporting zone while the portion of the luminescent medium adjacent to the cathode forms an electron injecting and transporting zone. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, a Frenkel exciton is formed. Recombination of this short-lived state may be visualized as an electron dropping from its conduction potential to a valence band, with relaxation occurring preferentially, under certain conditions, via a photoemissive mechanism. Under this view of the mechanism of operation of typical thin-layer organic devices, the electroluminescent layer comprises a luminescence zone receiving mobile charge carriers (electrons and holes) from each electrode.
A specific example of a red-emitting device is disclosed in U.S. Pat. No. 5,409,783 to Chang et al. The disclosed device utilizes an EL layer of tris(8-quinolinol) aluminum doped with magnesium phthalocyanine to achieve a red emission suitable for use in printing on color photographic paper. However, according to FIG. 5 of the reference, the emission spectrum of such a device, while displaying an emission maximum at approximately 690 nm, also displays a significant emission maximum at 530 nm (green). Although this apparently does not pose a problem for applications such as color printing, mentioned in Example 1 of the reference, such an emission would be unacceptable for use in a pixel device for a visual display in that the red light emission would be highly unsaturated.